1. Field of the Invention
The present invention relates to the field of photographic printers. Additionally, the invention is concerned with a lamphouse having a lamp in which a significantly greater percentage of the spectral make-up of the light beam therefrom is contained within the visible band as compared to a conventional tungsten lamp, and having light filters which can be shifted over a range of positions for selectively varying the respective intensities of the spectral components of the light beam.
2. Description of the Prior Art
A typical photographic printer of the prior art includes a lamphouse from which a beam of light is directed through photographic film onto photographic paper for producing a print. For conventional color prints, the photographic paper includes three layers of photosensitive dyes each of which responds to a different primary color, i.e., red, green and blue. Additionally, each dye responds with a different sensitivity to its respective primary color. To produce a high quality photograph, the printer exposes the dyes to the primary colors of the light beam for different lengths of time or different intensities in order to compensate for the different sensitivities of the dyes, for under or over exposure of the negative, and for variations in the dyes from one batch of paper to the next.
Three light filters cyan, magenta and yellow are included in the lamphouse to control the spectral make-up of the light beam emanating therefrom in order to achieve the desired exposure of the photographic paper to the three primary colors. A typical light filter is in the form of a "filter flag" comprised of a sheet of filter material extending from a solenoid controlled arm. Each light filter is operable to subtract or filter a primary color from the light beam, that is, a respective red, green or blue fractional portion the visible spectrum. This is known as subtractive or variable time mode printing.
During the first part of an exposure, unfiltered light, that is, white light, is directed through the negative onto the photographic paper in order to expose all of the dyes to the full spectrum. At the end of this time, the solenoid controlling the first filter is activated and this filter drops into the path of the beam in order to filter its portion of the spectrum from the beam. After an additional time, the second filter drops into place and an additional portion of the spectrum is subtracted. At the end of the exposure time, the third filter drops into place which subtracts the remaining portion of the spectrum from the beam which marks the end of the exposure time.
The typical prior art lamphouse of the printer described above uses an incandescent lamp having a tungsten filament. Use of such a lamp presents a number of problems because only about ten percent of the light output is in the visible band with the remaining ninety percent being in the infrared band among other losses. In addition to representing low power efficiency, the low level of visible light output requires longer exposure times which results in low productivity. Furthermore, the heat generated by the infrared must be dissipated so that the film negative and photographic paper are not damaged which requires ventilation and heat sink construction of the lamphouse adding to its cost. Increasing the wattage of the lamp is not a practical solution for increasing the intensity of the usable visible light because this also increases the amount of heat generated and merely adds to the inefficiency of the unit.
The spectral make-up of the visible light generated by the prior art incandescent lamp also represents inefficiency because the photographic paper dyes present peaks of sensitivity in their primary color portion of the spectrum, but the output of the lamp is continuous across the visible band. Additionally, the sensitivities of the dyes present cross-over areas which are affected by the generally uniform lamp output over the visible band.
The prior art filter flags also present problems because they experience "bounce" when they drop into position. During the bounce, unfiltered light leaks through which alters the intended spectral make-up of the beam and thereby affects the exposure. This problem becomes more acute with short exposure times in the range of 300 milliseconds, for example.
The prior art lamphouse described above also presents limited utility because it can only be operated in the "subtractive" mode. This means that each primary color is delivered at full intensity for part of the exposure time and then filtered, i.e., subtracted, entirely from the beam by the associated flag filter for the remainder of the exposure time. This results in an "average" level of exposure for each primary color, but such does not present the same photographic results as "additive" or variable intensity operation which exposes the dyes to all three colors at differing intensities for the entire exposure time. This difference comes about because of the time sensitivities of the dyes and because of the cross-over characteristics of dye sensitivities.
One prior art lamphouse which can operate in an additive mode uses three white light sources. Each light source is filtered so that the output represents a primary color which is then trimmed using trim filters to produce the desired level of primary color intensity. The respective primary color outputs are then combined into a single beam. As can be appreciated, the additive lamphouses are even less power efficient than subtractive lamphouses because only one-third of the visible spectrum from each lamp is used. Additionally, the trim filters often cannot be adjusted quickly and reliably enough for efficient production.
Another prior art lamphouse is disclosed in U.S. Pat. No. 3,754,824 which is concerned with a lamphouse having three dichroic filters which are operable to partially intercept a white light beam. These filters are manually adjustable and use a cam arrangement to produce a selected filter position. The filters engage the light beam from one side and can be progressively inserted in the beam to increase the filtering action. When low spectral component intensity is selected, only a small sliver of the periphery of the beam passes by the filter unattenuated. This presents a problem, however, in that the intensity of the beam diminishes outwardly from the center in a nonlinear fashion and as a result, it is very difficult to control and repeat a desired intensity at very low levels.